Low-pressure mercury arc discharge lamps, particularly of the fluorescent type having an emission spectrum primarily in the ultraviolet (U.V.) region, have been employed in the biomedical treatment of skin disorders. However, since these lamps emit a relatively broad spectrum of ultraviolet radiation, including both long wavelength U.V. (about 340 nm) and short wavelength U.V. (below 340 nm); and since in some biomedical applications the use of short wavelength U.V. is contra-indicated, the application of these lamps has been restricted. The short wavelength U.V. emissions are due, primarily, to the low-pressure mercury arc discharge itself, which has an emission peak at 313 nm, and secondarily, from the shorter wavelength emissions emanating from a phosphor coating which can be disposed within the lamp.
Prior art suggestions attempting to solve these problems have involved internal and external U.V. filtering systems, examples of which are disclosed in U.S. Pat. Nos. 3,541,376; 3,720,826; and 4,048,537. Such filtering systems tend to be expensive and, furthermore, they are non-selective in the U.V. range of 300 nm to 400 nm, ranging from high transmission at all U.V. wavelengths to very low transmission at all U.V. wavelengths.